user: Move 'thunk.h' from 'exec/user' to 'user'

Keep all user emulation headers under the same user/ directory.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20240428221450.26460-2-philmd@linaro.org>
5 files changed